Audio/Video Transport Core A. Williams
Maintenance Audinate
Internet-Draft K. Gross
Intended status: Standards Track AVA Networks
Expires: September 26, 2014 R. van Brandenburg
H. Stokking
TNO
March 25, 2014
RTP Clock Source Signallingdraft-ietf-avtcore-clksrc-11
Abstract
NTP format timestamps are used by several RTP protocols for
synchronisation and statistical measurements. This memo specifies
SDP signalling identifying timestamp reference clock sources and SDP
signalling identifying the media clock sources in a multimedia
session.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 26, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Internet-Draft RTP Clock Source Signalling March 20141. Introduction
RTP protocols use NTP format timestamps to facilitate multimedia
session synchronisation and for providing estimates of round trip
time (RTT) and other statistical parameters.
Information about media clock timing exchanged in NTP format
timestamps may come from a clock which is synchronised to a global
time reference, but this cannot be assumed nor is there a
standardised mechanism available to indicate that timestamps are
derived from a common reference clock. Therefore, RTP
implementations typically assume that NTP timestamps are taken using
unsynchronised clocks and must compensate for absolute time
differences and rate differences. Without a shared reference clock,
RTP can time align flows from the same source at a given receiver
using relative timing, however tight synchronisation between two or
more different receivers (possibly with different network paths) or
between two or more senders is not possible.
High performance AV systems often use a reference media clock
distributed to all devices in the system. The reference media clock
is often distinct from the reference clock used to provide
timestamps. A reference media clock may be provided along with an
audio or video signal interface, or via a dedicated clock signal
(e.g. genlock [SMPTE-318-1999] or audio word clock [AES11-2009]). If
sending and receiving media clocks are known to be synchronised to a
common reference clock, performance can improved by minimising
buffering and avoiding rate conversion.
This specification defines SDP signalling of timestamp reference
clock sources and media reference clock sources.
2. Applications
Timestamp reference clock source and media clock signalling benefit
applications requiring synchronised media capture or playout and low
latency operation.
Examples include, but are not limited to:
Social TV : RTCP for inter-destination media synchronization
[I-D.ietf-avtcore-idms] defines social TV as the combination of
media content consumption by two or more users at different
devices and locations and real-time communication between those
users. An example of Social TV, is where two or more users are
watching the same television broadcast at different devices and/or
locations, while communicating with each other using text, audio
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and/or video. A skew in the media playout of the two or more
users can have adverse effects on their experience. A well-known
use case here is one friend experiencing a goal in a football
match well before or after other friends.
Video Walls : A video wall consists of multiple computer monitors,
video projectors, or television sets tiled together contiguously
or overlapped in order to form one large screen. Each of the
screens reproduces a portion of the larger picture. In some
implementations, each screen or projector may be individually
connected to the network and receive its portion of the overall
image from a network-connected video server or video scaler.
Screens are refreshed at 50 or 60 hertz or potentially faster. If
the refresh is not synchronized, the effect of multiple screens
acting as one is broken.
Networked Audio : Networked loudspeakers, amplifiers and analogue
I/O devices transmitting or receiving audio signals via RTP can be
connected to various parts of a building or campus network. Such
situations can for example be found in large conference rooms,
legislative chambers, classrooms (especially those supporting
distance learning) and other large-scale environments such as
stadiums. Since humans are more susceptible to differences in
audio delay, this use case needs even more accuracy than the video
wall use case. Depending on the exact application, the need for
accuracy can then be in the range of microseconds [Olsen].
Sensor Arrays : Sensor arrays contain many synchronised measurement
elements producing signals which are then combined to form an
overall measurement. Accurate capture of the phase relationships
between the various signals arriving at each element of the array
is critically important for proper operation. Examples include
towed or fixed sonar arrays, seismic arrays and phased arrays used
in radar applications, for instance.
3. Definitions
The following definitions are used in this draft:
media level : Media level information applies to a single SDP media
stream. In an SDP description, media-level information appears
after each "m"-line.
multimedia session : A set of multimedia senders and receivers as
well as the data streams flowing from senders to receivers. The
Session Description Protocol (SDP) [RFC4566] describes multimedia
sessions.
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RTP media stream : A single stream of RTP packets identified by an
RTP SSRC.
RTP media sender : The device generating an associated RTP media
stream
SDP media stream : An RTP session potentially containing more than
one RTP source. SDP media descriptions beginning with an "m"-line
define the parameters of an SDP media stream.
session level : Session level information applies to an entire
multimedia session. In an SDP description, session-level
information appears before the first "m"-line.
source level : Source level information applies to a specific RTP
media stream. Source-Specific Media Attributes in the Session
Description Protocol (SDP) [RFC5576] defines how source-level
information is included into an SDP session description.
traceable time : A clock is considered to provide traceable time if
it can be proven to be synchronised to International Atomic Time
(TAI). Coordinated Universal Time (UTC) is a time standard
synchronized to TAI. UTC is therefore also considered traceable
time once leap seconds have been taken unto account. GPS
[IS-GPS-200F] is commonly used to provide a TAI traceable time
reference. Some network time synchronisation protocols (e.g. PTP
[IEEE1588-2008], NTP) can explicitly indicate that the master
clock is providing a traceable time reference over the network.
4. Timestamp Reference Clock Source Signalling
The NTP format timestamps used by RTP are taken by reading a local
real-time clock at the sender or receiver. This local clock may be
synchronised to another clock (time source) by some means or it may
be unsynchronised. A variety of methods are available to synchronise
local clocks to a reference time source, including network time
protocols (e.g. NTP [RFC5905], PTP [IEEE1588-2008]) and radio clocks
(e.g. GPS [IS-GPS-200F]).
The following sections describe and define SDP signalling, indicating
whether and how the local timestamping clock in an RTP sender/
receiver is synchronised to a reference clock.
4.1. Clock synchronization
Two or more local clocks that are sufficiently synchronised will
produce timestamps for a given RTP event can be used as if they came
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from the same clock. Providing they are sufficiently synchronised,
timestamps produced in one RTP sender or receiver can be directly
compared to a local clock in another RTP sender or receiver.
The accuracy of synchronisation required is application dependent.
See Applications (Section 2) section for a discussion of applications
and their corresponding requirements. To serve as a reference clock,
clocks must minimally be syntonized (exactly frequency matched) to
one another.
Sufficient synchronisation can typically be achieving by using a
network time protocol (e.g. NTP, 802.1AS, IEEE 1588-2008) to
synchronize all devices to a single master clock.
Another approach is to use clocks providing a global time reference
(e.g. GPS, Galileo, GLONASS). This concept may be used in
conjunction with network time protocols as some protocols (e.g. PTP,
NTP) allow master clocks to indicate explicitly that they are
providing traceable time.
4.2. Identifying NTP Reference Clocks
A single NTP server is identified by hostname (or IP address) and an
optional port number. If the port number is not indicated, it is
assumed to be the standard NTP port (123).
Two or more NTP servers MAY be listed at the same level in the
session description to indicate that all of the listed servers
deliver the same reference time and may be used interchangeably. RTP
senders and receivers are assured proper synchronization regardless
of which server they choose and, in support of fault tolerance, may
switch servers while streaming.
4.3. Identifying PTP Reference Clocks
The IEEE 1588 Precision Time Protocol (PTP) family of clock
synchronisation protocols provides a shared reference clock in an
network - typically a LAN. IEEE 1588 provides sub-microsecond
synchronisation between devices on a LAN and typically locks within
seconds at startup. With support from Ethernet switches, IEEE 1588
protocols can achieve nanosecond timing accuracy in LANs. Network
interface chips and cards supporting hardware time-stamping of timing
critical protocol messages are also available.
Three flavours of IEEE 1588 are in use today:
o IEEE 1588-2002 [IEEE1588-2002]: the original "Standard for a
Precision Clock Synchronization Protocol for Networked Measurement
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and Control Systems". This is also known as IEEE1588v1 or PTPv1.
o IEEE 1588-2008 [IEEE1588-2008]: the second version of the
"Standard for a Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems". This is a revised
version of the original IEEE1588-2002 standard and is also known
as IEEE1588v2 or PTPv2. IEEE 1588-2008 is not protocol compatible
with IEEE 1588-2002.
o IEEE 802.1AS [IEEE802.1AS-2011]: "Timing and Synchronization for
Time Sensitive Applications in Bridged Local Area Networks". This
is a Layer-2 only profile of IEEE 1588-2008 for use in Audio/Video
Bridged LANs as described in IEEE 802.1BA-2011 [IEEE802.1BA-2011].
Each IEEE 1588 clock is identified by an EUI-64 called a
"ClockIdentity". A slave clock using one of the IEEE 1588 family of
network time protocols acquires the ClockIdentity/EUI-64 of the
grandmaster clock that is the ultimate source of timing information
for the network. A boundary clock which is itself slaved to another
boundary clock or the grandmaster passes the grandmaster
ClockIdentity through to its slaves.
Several instances of the IEEE 1588 protocol may operate independently
on a single network, forming distinct PTP domains, each of which may
have a different grandmaster clock. As the IEEE 1588 standards have
developed, the definition of PTP domains has changed. IEEE 1588-2002
identifies protocol subdomains by a textual name, but IEEE 1588-2008
identifies protocol domains using a numeric domain number. 802.1AS is
a Layer-2 profile of IEEE 1588-2008 supporting a single numeric clock
domain (0).
When PTP domains are signalled via SDP, senders and receivers SHOULD
check that both grandmaster ClockIdentity and PTP domain match when
determining clock equivalence.
Two or more IEEE 1588 clocks MAY be listed at the same level in the
session description to indicate that all of the listed clocks are
candidate grandmaster clocks for the domain or deliver the same
reference time and may be used interchangeably. RTP senders and
receivers are assured proper synchronization regardless of which
synchronization source they choose and, in support of fault
tolerance, may switch reference clock source while streaming.
The PTP protocols employ a distributed election protocol called the
"Best Master Clock Algorithm" (BMCA) to determine the active clock
master. The clock master choices available to BMCA can be restricted
or biased by configuration parameters to influence the election
process. In some systems it may be desirable to limit the number of
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possible PTP clock masters to avoid the need to re-signal timestamp
reference clock sources when the clock master changes.
4.4. Identifying Global Reference Clocks
Global reference clocks provide a source of traceable time, typically
via a hardware radio receiver interface. Examples include GPS,
Galileo and GLONASS. Apart from the name of the reference clock
system, no further identification is required.
4.5. Private Reference Clocks
In other systems, all RTP senders and receivers may use a timestamp
reference clock that is not provided by one of the methods listed
above. Examples may include the reference time information provided
by digital television or cellular services. These sources are
identified as "private" reference clocks. All RTP senders and
receivers in a session using a private reference clock are assumed to
have a mechanism outside this specification for determining whether
their timestamp reference clocks are equivalent.
4.6. Local Reference ClocksRFC 3550 allows senders and receivers to either use a local wall
clock reference for their NTP timestamps or, by setting the timestamp
field to 0, to supply no timestamps at all. Both are common practice
in embedded RTP implementations. These clocks are identified as
"local" and can only be assumed to be equivalent to clocks
originating from the same device.
4.7. Traceable Reference Clocks
A timestamp reference clock source may be labelled "traceable" if it
is known to be to delivering traceable time. Providing adjustments
are made for differing epochs, timezones and leap seconds, timestamps
taken using clocks synchronised to a traceable time source can be
directly compared even if the clocks are synchronised to different
sources or via different mechanisms.
Marking a clock as traceable allows additional information (e.g. IP
addresses, PTP master identifiers and the like) to be omitted from
the SDP since any traceable clock available at the answerer is
considered to be an appropriate timestamp reference clock. For
example, an offerer could could specify ts-refclk:ntp=/traceable/ and
the answerer could use GPS as a reference clock since GPS is a source
of traceable time.
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Specification of the timestamp reference clock source may be at any
or all levels (session, media or source) of an SDP description (see
level definitions (Section 3) earlier in this document for more
information).
Timestamp reference clock source signalling included at session-level
provides default parameters for all RTP sessions and sources in the
session description. More specific signalling included at the media
level overrides default session level signalling. More specific
signalling included at the source level overrides default media level
signalling.
If timestamp reference clock source signalling is included anywhere
in an SDP description, it must be properly defined for all levels in
the description. This may simply be achieved by providing default
signalling at the session level.
Timestamp reference clock parameters may be repeated at a given level
(i.e. for a session or source) to provide information about
additional servers or clock sources. If the attribute is repeated at
a given level, all clocks described at that level are assumed to be
equivalent. Traceable time sources MUST NOT be mixed with non-
traceable time sources at any given level.
Note that clock source parameters may change from time to time, for
example, as a result of a PTP clock master election. The SIP
[RFC3261] protocol supports re-signalling of updated SDP information,
however other protocols may require additional notification
mechanisms.
General forms of usage:
session level: a=ts-refclk:<clksrc>
media level: a=ts-refclk:<clksrc>
source level: a=ssrc:<ssrc-id> ts-refclk:<clksrc>
ABNF [RFC5234] grammar for the timestamp reference clock attribute:
; external references:
POS-DIGIT = <See RFC 4566>
token = <See RFC 4566>
byte-string = <See RFC 4566>
DIGIT = <See RFC 5324>
HEXDIG = <See RFC 5324>
CRLF = <See RFC 5324>
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v=0
o=jdoe 2890844526 2890842807 IN IP4 192.0.2.1
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 233.252.0.1/64
t=2873397496 2873404696
a=recvonly
a=ts-refclk:local
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000
a=ssrc:12345 ts-refclk:ptp=IEEE802.1AS-2011:39-A7-94-FF-FE-07-CB-D0
Figure 4: Timestamp reference clock signalling at the source level
5. Media Clock Source Signalling
The media clock source for a stream determines the timebase used to
advance the RTP timestamps included in RTP packets. The media clock
may be asynchronously generated by the sender, it may be generated in
fixed relationship to the reference clock or it may be generated with
respect to another stream on the network (which is presumably being
received by the sender).
5.1. Asynchronously Generated Media Clock
In the simplest sender implementation, the sender generates media by
sampling audio or video according to a free-running local clock. The
RTP timestamps in media packets are advanced according to this media
clock and packet transmission is typically timed to regular intervals
on this timeline. The sender may or may not include an NTP timestamp
in sender reports to allow mapping of this asynchronous media clock
to a reference clock.
The asynchronously generated media clock is the assumed mode of
operation when there is no signalling of media clock source.
Alternatively, asynchronous media clock may be explicitly signalled.
a=mediaclk:sender
5.2. Direct-Referenced Media Clock
A media clock may be directly derived from a reference clock. For
this case it is required that a reference clock be specified with an
a=ts-refclk attribute (Section 4.8).
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The signalling optionally indicates a media clock offset value. The
offset indicates the RTP timestamp value at the epoch (time of
origin) of the reference clock. To use the offset, implementations
need to compute RTP timestamps from reference clocks. To simplify
these calculations, streams utilizing offset signalling SHOULD use a
TAI timestamp reference clock to avoid complications introduced by
leap seconds. See [I-D.ietf-avtcore-leap-second] for further
discussion of leap-second issues in timestamp reference clocks.
To compute the RTP timestamp against an IEEE 1588 (TAI-based)
reference, the time elapsed between the 00:00:00 1 January 1970 IEEE
1588 epoch and the current time must be computed. Between the epoch
and 1 January 2013, there were 15,706 days (including extra days
during leap years). Since there are no leap seconds in a TAI
reference, there are exactly 86,400 seconds during each of these days
or a total of 1,356,998,400 seconds from the epoch to 00:00:00 1
January 2013. A 90 kHz RTP clock for a video stream would have
advanced 122,129,856,000,000 units over this period. With a
signalled offset of 0, the RTP clock value modulo the 32-bit unsigned
representation in the RTP header would have been 2,460,938,240 at 00:
00:00 1 January 2013. If an offset of 23,465 had been signalled, the
clock value would have been 2,460,961,705.
In order to use an NTP reference, the actual time elapsed between the
00:00:00, 1 January 1900 NTP epoch to the current time must be
computed. 2,208,988,800 seconds elapsed between the NTP epoch and 00:
00:00 1 January 1970 [RFC0868]. Between the beginning of 1970 and
2013, there were 15,706 days elapsed (including extra days during
leap years) and 25 leap seconds inserted. There is therefore a total
of 3,565,987,225 seconds from the NTP epoch to 00:00:00 1 January
2013. A 90 kHz RTP clock for a video stream would have advanced
320,938,850,250,000 units over this period. With a signalled offset
of 0, the RTP clock value modulo the 32-bit unsigned representation
would have been 1,714,023,696 at 00:00:00 1 January 2013.
If no offset is signalled, the offset can be inferred at the receiver
by examining RTCP sender reports which contain NTP and RTP timestamps
which combined define a mapping. The NTP/RTP timestamp mapping
provided by RTCP SRs takes precedence over that singaled through SDP,
however the media clock rate implied by the SRs MUST be consistent
with the rate signalled.
A rate modifier may be specified. The modifier is expressed as the
ratio of two integers and modifies the rate specified or implied by
the media description by this ratio. If omitted, the rate is assumed
to be the exact rate specified or implied by the media format. For
example, without a rate specification, the RTP clock for an 8 kHz
G.711 audio stream will advance exactly 8000 units for each second
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advance in the reference clock from which it is derived.
The rate modifier is primarily useful for accommodating certain
"oddball" audio sample rates associated with NTSC video (see
Figure 7). Modified rates are not advised for video streams which
generally use a 90 kHz RTP clock regardless of frame rate or sample
rate used for embedded audio.
a=mediaclk:direct[=<offset>] [rate=<rate numerator>/<rate
denominator>]
5.3. Stream-Referenced Media Clock
A common synchronisation architecture for audio/visual systems
involves distributing a reference media clock from a master device to
a number of slave devices, typically by means of a cable. Examples
include audio word clock distribution and video black burst
distribution. In this case, the media clock is locally generated,
often by a crystal oscillator and is not locked to a timestamp
reference clock.
To support this architecture across a network, a master clock
identifier is associated with an RTP media stream carrying media
clock timing information from a master device. The master clock
identifier represents a media clock source in the master device.
Slave devices in turn associate the master media clock identifier
with streams they transmit, signalling the synchronisation
relationship between the master and the transmitter's media clock.
Slave devices recover media clock timing from the clock master
stream, using it to synchronise the slave media clock with the
master. Timestamps in the master clock RTP media stream are taken
using the timestamp reference clock shared by the master and slave
devices. The timestamps communicate information about media clock
timing (rate, phase) from the master to the slave devices.
Timestamps are communicated in the usual RTP fashion via RTCP SRs, or
via the RFC6051 [RFC6051] header extension. The stream media format
may indicate other clock information, such as the nominal rate.
Note that slaving of a device media clock to a master device does not
affect the usual RTP lip sync / time alignment algorithms. Time
aligned playout of two or more RTP sources still relies upon NTP
timestamps supplied via RTCP SRs or by the RFC6051 timestamp header
extension.
In a given system, master clock identifiers must uniquely identify a
single media clock source. Such identifiers MAY be manually
configured, however identifiers SHOULD be generated according to the
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"short-term persistent RTCP CNAME" algorithm as described in RFC7022
[RFC7022]. Master clock identifiers not already in base64 format
MUST be encoded as a base64 strings when used in SDP. Although the
RTCP CNAME algorithm is used to generate the master clock identifier,
it is used to tag RTP sources in SDP descriptions and does not appear
in RTCP as a CNAME.
A reference stream can be an RTP stream or AVB stream based on the
IEEE 1722 [IEEE1722] standard.
An RTP clock master stream SHOULD be identified at the source level
by an SSRC [RFC5576] and master clock identifier. An RTP stream that
provides media clock timing directly from a reference media clock
(e.g. internal crystal, audio word clock or video blackburst signal)
SHOULD tag the stream as a master clock source using the "src:"
prefix. If master clock identifiers are declared at the media or
session level, all RTP sources at or below the level of declaration
MUST provide equivalent timing to a slave receiver.
a=ssrc:<ssrc> mediaclk:id=src:<media-clktag> sender
a=mediaclk:id=src:<media-clktag> sender
A transmitted RTP stream slaved to media clock master is signalled by
including master clock identifier:
a=mediaclk:id=<media-clktag> sender
An RTP media sender indicates that it is slaved to an IEEE 1722 clock
master via a stream identifier (an EUI-64):
a=mediaclk:IEEE1722=<StreamID>
An RTP media sender may gateway IEEE 1722 media clock timing to RTP:
a=mediaclk:id=src:<media-clktag> IEEE1722=<StreamID>
5.4. SDP Signalling of Media Clock Source
Specification of the media clock source may be at any or all levels
(session, media or source) of an SDP description (see level
definitions (Section 3) earlier in this document for more
information).
Media clock source signalling included at session level provides
default parameters for all RTP sessions and sources in the session
description. More specific signalling included at the media level
overrides default session level signalling. Further, source-level
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signalling overrides media clock source signalling at the enclosing
media level and session level.
Media clock source signalling may be present or absent on a per-
stream basis. In the absence of media clock source signals,
receivers assume an asynchronous media clock generated by the sender.
Media clock source parameters may be repeated at a given level (i.e.
for a session or source) to provide information about additional
clock sources. If the attribute is repeated at a given level, all
clocks described at that level are comparable clock sources and may
be used interchangeably.
General forms of usage:
session level: a=mediaclk:<mediaclock>
media level: a=mediaclk:<mediaclock>
source level: a=ssrc:<ssrc-id> mediaclk:<mediaclock>
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v=0
o=- 1311738121 1311738121 IN IP4 192.0.2.1
c=IN IP4 233.252.0.1/64
s=
t=0 0
m=audio 5004 RTP/AVP 96
a=rtpmap:96 L24/48000/2
a=sendonly
a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
a=mediaclk:IEEE1722=38-D6-6D-8E-D2-78-13-2F
Figure 9: RTP stream with media clock slaved to an IEEE1722 master
device
6. Signalling Considerations
Signalling of timestamp reference clock source (Section 4.8) and
media clock source (Section 5.4) is defined to be used either by
applications that implement the SDP Offer/Answer model [RFC3264] or
by applications that use SDP to describe media and transport
configurations.
A description SHOULD include both reference clock signalling and
media clock signalling. If no reference clock is available, this
SHOULD be signalled as a local reference (Section 4.6).
When no media clock signalling is present, an asynchronous media
clock (Section 5.1) MUST be assumed. When no reference clock
signalling is present, a local reference clock (Section 4.6) MUST be
assumed.
If a reference clock is not signalled or a local reference is
specified, the corresponding media clock may be established as rate
synchronised with no assurance of time synchronisation.
When the description signals a direct-referenced media clock
(Section 5.2), reference clock signalling is REQUIRED. Asynchronous
and stream-referenced media clocks (Section 5.3) MAY be specified
with or without a reference clock signalling.
6.1. Usage in Offer/Answer
During offer/answer, clock source signalling via SDP uses a
declarative model. Supported media and/or reference clocks are
specified in the offered SDP description. The answerer may accept or
reject the offer in an application-specific way depending on the
clocks that are available and the clocks that are offered. For
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example, an answerer may choose to accept an offer that lacks a
common clock by falling back to a lower performance mode of operation
(e.g. by assuming reference or media clocks are local rather than
shared). Conversely, the answerer may choose to reject the offer
when the offered clock specifications indicate that the available
reference and/or media clocks are incompatible.
While negotiation of reference clock and media clock attributes is
not defined in this document, negotiation MAY be accomplished using
the capabilities negotiation procedures defined in [RFC5939].
6.1.1. Indicating Support for Clock Source Signalling
An offerer or answerer indicates support for media clock signalling
by including a reference or media clock specification in the SDP
description. An offerer or answerer without specific reference or
media clocks to signal SHOULD indicate support for clock source
signalling by including a local reference clock (Section 4.6)
specification in the SDP description.
6.1.2. Timestamp Reference Clock
If one or more of the reference clocks specified in the offer are
usable by the answerer, the answerer SHOULD respond with an answer
containing the subset of reference clock specifications in the offer
that are usable by the answerer. If the answerer rejects the offer
because the available reference clocks are incompatible, the
rejection MUST contain at least one timestamp reference clock
specification usable by the answerer so that appropriate information
is available for debugging. If no external reference clock is
available to the answerer a local reference clock (Section 4.6)
specification SHOULD be included in the rejection.
In both offers and answers, multiple reference clock specifications
indicate equivalent clocks from different sources which may be used
interchangeably. RTP senders and receivers are assured proper
synchronization regardless of which of the specified sources is
chosen and, in support of fault tolerance, may switch clock sources
while streaming.
6.1.3. Media Clock
If the media clock mode specified in the offer is acceptable to the
answerer, the answerer SHOULD respond with an answer containing the
same media clock specification as the offer. If the answerer rejects
the offer because the available reference clocks are incompatible,
the rejection MUST contain a media clock specification supported by
the answerer so that appropriate information is available for
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Internet-Draft RTP Clock Source Signalling March 2014
debugging. If no shared media clocks are available to the answerer
an asynchronous media clock (Section 5.1) specification SHOULD be
included in the rejection.
6.2. Usage Outside of Offer/Answer
SDP can be employed outside of the Offer/Answer context, for instance
for multimedia sessions that are announced through the Session
Announcement Protocol (SAP) [RFC2974], or streamed through the Real
Time Streaming Protocol (RTSP) [RFC2326].
Devices using published descriptions to join sessions SHOULD assess
their synchronization compatibility with the described session based
on the clock source signalling and SHOULD NOT attempt to join a
session with incompatible reference or media clocks.
7. Security Considerations
Entities receiving and acting upon an SDP message should note that a
session description cannot be trusted unless it has been obtained by
an authenticated transport protocol from a known and trusted source.
Many different transport protocols may be used to distribute session
description, and the nature of the authentication will differ from
transport to transport. For some transports, security features are
often not deployed. In case a session description has not been
obtained in a trusted manner, the endpoint should exercise care
because, among other attacks, the media sessions received may not be
the intended ones, the destination where media is sent to may not be
the expected one, any of the parameters of the session may be
incorrect.
Incorrect reference or media clock parameters may cause devices or
streams to synchronize to unintended clock sources. Normally this
simply results in failure to establish a session or failure to
synchronize once connected. Enough devices fraudulently assigned to
a specific clock source (e.g. a particular IEEE 1588 grandmaster)
may, however, constitute a successful denial of service attack on
that source. Devices MAY wish to validate the integrity of the clock
description through some means before connecting to unfamiliar clock
sources.
The timestamp reference clocks negotiated by this protocol are used
to provide media timing information to RTP. Negotiated timestamp
reference clocks should not be relied upon to provide a secure time
reference for security critical operations (e.g. the expiration of
public key certificates).
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Internet-Draft RTP Clock Source Signalling March 20148. IANA Considerations
This document defines two new SDP attributes: 'ts-refclk' and
'mediaclk', within the existing Internet Assigned Numbers Authority
(IANA) registry of SDP Parameters.
This document also defines a new IANA registry subordinate to the
IANA SDP Parameters registry: the Media Clock Source Parameters
Registry. Within this new registry, this document defines an initial
set of three media clock source parameters. Further, this document
defines a second new IANA registry subordinate to the IANA SDP
Parameters registry: the Timestamp Reference Clock Source Parameters
Registry. Within this new registry, this document defines an initial
six parameters.
8.1. Reference Clock SDP Parameter
The SDP attribute "ts-refclk" defined by this document is registered
with the IANA registry of SDP Parameters as follows:
SDP Attributes ( "att-field (both session and media level)" &
"att-field (source level)" ):
Attribute name: ts-refclk
Long form: Timestamp reference clock source
Type of name: att-field
Type of attribute: Session, media and source level
Subject to charset: No
Purpose: See section 4 of this document
Reference: This document
Values: See section 8.3 of this document
Figure 10
The attribute has an extensible parameter field and therefore a
registry for these parameters is required. This new registry is
defined in Section 8.3.
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Internet-Draft RTP Clock Source Signalling March 20148.2. Media Clock SDP Parameter
The SDP attribute "mediaclk" defined by this document is registered
with the IANA registry of SDP Parameters as follows:
SDP Attributes ( "att-field (both session and media level)" &
"att-field (source level)" ):
Attribute name: mediaclk
Long form: Media clock source
Type of name: att-field
Type of attribute: Session, media and source level
Subject to charset: No
Purpose: See section 5 of this document
Reference: This document
Values: See section 8.4 of this document
Figure 11
The attribute has an extensible parameter field and therefore a
registry for these parameters is required. The new registry is
defined in Section 8.4.
8.3. Timestamp Reference Clock Source Parameters Registry
This document creates a new IANA sub-registry called the Timestamp
Reference Clock Source Parameters Registry, subordinate to the IANA
SDP Parameters registry. Each entry in the Timestamp Reference Clock
Source Parameters Registry contains:
Name: Token used in the SDP description (clksrc-param-name)
Long name: Descriptive name for the timestamp reference clock source
Reference: Reference to the document describing the SDP token
(clksrc-param-name) and syntax for the optional value associated
with the token (mediaclock-param-value)
Initial values for the Timestamp Reference Clock Source Parameters
registry are given below.
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